Marine litter and ocean currents

When Ryan Robinson arrived on Home Island, Cocos Kealing in
October 2008, he realized that the problem of marine litter was much worse than
he had previously imagined. The plastic
he found on the uninhabited east coast of that island included 246 plastic
water bottles and 339 plastic sandals over a 100-metre stretch of beach.Robinson, along with brother Bryson and friend
Hugh Patterson had been sailing around the world aboard the 40-foot Khulula on a mission to collect and
catalogue trash found on remote shores. In
their three-year mission they failed to find a garbage-free beach, including on
some islands that had not been visited by humans for many years.

Once it finds its way into in the sea, plastic is carried
around the planet at the mercy of the ocean currents and surface winds. Even
before it gets a chance to be broken down into tiny pieces it gets transported
thousands of miles around the globe and sometimes ends up in places uninhabited
by humans. Robinson and crew
figured out that the trash they found on Home Island had been brought over 1000
km from Indonesia, riding on the equatorial current and the easterly tradewind
belt. Much of the plastic had writing in
Bahasa Indonesian; and amongst it were pieces of Indonesian bamboo, which does
not grow on Cocos Kealing.

Plastic trash is transported all over the planet by the
ocean currents. The power of the currents to transport plastic objects was
demonstrated in a famous accident-turned-experiment when, in January 1992, over
28,000 plastic bath toys washed off a ship in the North Pacific. Apart from the
inevitable yellow ducks, the consignment also contained red beavers, green
frogs and blue turtles. The plight of the plastic animals was famously studied
by oceanographer Curtis Ebbesmeyer, who not only monitored their landfall over
the next decade or so by contacting local coastal residents, but also predicted
their movements using a computer model of ocean surface currents.

In November 1992, ten months after they started their
journey, the animals began to wash up on the coast of Alaska, about 3000 km
from their starting point. Four years later, in 1996, more landed further south
in Washington State. Ebbesmeyer and co-workers predicted that many of them
would have also travelled up into the Arctic and become trapped in the ice.
There where they would remain for about five or six years, gradually drifting
from the North Pacific into the North Atlantic. The prediction was verified
when, between 2000 and 2003, more of the toys were washed ashore on the eastern
seaboard of North America.

It was also hypothesized that a huge number of the animals
would continue drifting around the North Pacific, caught up in the Gyres. The
Gyres are vast circulating areas of water containing the major surface currents
of the world, driven by the prevailing winds. The toys would have been caught
up in the North Pacific Subpolar and Subtropical Gyres, and many of them would
have ended up in an area in the northeast corner of the North Pacific Subtropical
Gyre, known as the Great Pacific Garbage
Patch.

The term ‘Great Pacific Garbage Patch’ was first coined by Captain
Charles Moore, the world’s best-known advocate against marine plastics. In
1997, while sailing across the northeast Pacific in his 50-foot catamaran,
Moore discovered a vast area of floating plastic. Even though NOAA scientists
had predicted a decade before that there would be a lot of marine debris in
that area, it was Captain Moore’s first-hand account that really helped to draw
the world’s attention to the problem of marine litter.

Moore found large intact items such as bottles, cups, bags,
boxes and fishing nets, but also millions of multi-coloured, unidentifiable
smaller pieces of plastic, over an area estimated to cover somewhere between 650,000 up to about 16,000,000 square
kilometres. Importantly, only
about 30 per cent of the total plastic is on the surface, while the other 70
per cent is under the surface and out of site.

‘Garbage patches’ are found in the world’s oceans wherever
there is a gyre. The Pacific is not the
only place in the world’s oceans where you will find a concentration of
plastic; ‘garbage patches’ also exist in the other four major gyres of the
oceans: in the North Atlantic, South Atlantic, South Pacific and Indian Oceans,
and in several smaller gyres such as the North Pacific Subpolar Gyre,
mentioned above.

The currents in the gyre suck in floating material from
around the periphery and trap it in the centre. The waves and currents break up the plastic into smaller and smaller
pieces, but it never goes away. The gyre is like a huge processing machine –
continuously taking in large pieces of plastic and grinding them up into
smaller pieces. But it’s a machine with no output, so it is gradually clogging
up with plastic.

It is really important
to stress that plastic just doesn’t go away. You might see charts showing
‘degrade times’ for various different plastic items (e.g. ‘plastic bag: 100
years’), but it is unclear where these estimates come from. Unfortunately it is overly optimistic to
state that plastic will revert back to the natural environment within any
reasonable timescale. It doesn’t
biodegrade because, since it was only invented less than a century ago, no bacteria
capable of eating it have had a chance to evolve yet. A tiny amount might photodegrade on the
surface of landfills in hot climates, but most of the plastic in the sea is
well out of the way of sunlight. In
fact, virtually all the plastic ever manufactured since it was invented still exists.

To illustrate this
point, a piece of
plastic found in the stomach of an albatross around 2005 contained a serial
number that was subsequently traced to a World War II seaplane shot down in 1944. That piece of
plastic was one of the first pieces of plastic ever produced, and it survived
until the present day. Once
plastic ends up in the sea, it will drift around until it ends up on some
remote shoreline, or it will simply drift around until a plastic-eating bacterium
evolves – perhaps a few hundred thousand years.

The ocean currents are one of the most important mechanisms
of the working planet. They transport a colossal amount of water around the
globe. For example, the Gulf Stream, the most well-known current to most of us,
has a volumetric flow rate of up to 120 million cubic metres per second – over
100 times the combined flow of all the rivers in the world. The velocity of the
Gulf Stream can reach up to five knots – enough to significantly alter the
ground speed of any vessel sailing through it.

The ocean currents carry enormous amounts of heat around the
planet. The uneven input of solar energy at the poles compared to the equator
means that a heat redistribution system must exist on the planet; otherwise the
temperature difference between the poles and the equator would spiral out of
control. Some of that heat redistribution is achieved by the large-scale
atmospheric circulation; but a large proportion is achieved by the ocean
currents.

Water has such a high specific heat capacity that it can
store and transport a tremendous amount of heat energy for a relatively small
increase in its own temperature. For example, the amount of heat energy
transported by the Gulf Stream is in the order of petawatts (1015 W)
– equivalent to a million 1-GW power stations.

Because of their vast heat-carrying capacity, the ocean
currents have a major influence on local climates. Countries such as Ireland tend
to have warmer and wetter climates than usual, because they are at the
receiving end of warm currents like the Gulf Stream. In contrast, areas such as
coastal Namibia have colder and drier climates than usual, because they are situated
near cold currents like the Benguela Current.

Currents also have a biological role. The availability of nutrients in a particular
zone is affected by local changes in water temperature and salinity due to the
currents. This is important for marine
plants, and hence for the entire local ecosystem. The currents also act as conveyer belts for
marine animals themselves; for example by transporting eggs from the place
where they were laid to the place where they will hatch, each place requiring
different environmental conditions.

The ocean surface currents are driven principally by the
wind. Energy is transferred from the overlying air to the surface of the water.
Most of this energy ends up as waves, but a lot of it also ends up as
currents. In areas where there are
stable wind patterns, the force of the wind literally pushes the surface water
along, creating surface currents systems that coincide with the prevailing wind
patterns.

The prevailing wind patterns include large
anticyclones where the wind constantly blows around an area of high pressure.
These anticyclones are more or less stable systems (termed quasi-stable), with the surface winds blowing over more or less the
same areas of ocean all the time. The ocean surface currents mimic the
overlying wind patterns, themselves circulating in an anticyclonic direction
(clockwise in the northern hemisphere and anticlockwise in the southern). They
have an area of calm water in the middle just like the area of calm winds at
the centre of an anticyclone. These are the major oceanic gyres of the world,
which coincide with the five major anticyclones, in the north and south
Pacific, north and south Atlantic and in the Indian Ocean (Figure 1). Since the water in the centre of the gyre is very
calm, any floating object trapped there will likely stay there for a long time.

Figure 1: major gyres of the
world

The ocean currents evolved to have a function as part of the
living planet. They transport ‘useful stuff’ like heat energy, organic
nutrients and marine animals from one part of the globe to the other. However, currents
can also transport anything else that happens to be floating on the ocean
surface, including not-so-useful stuff like plastic trash. The plastic,
particularly larger pieces that stick up from the surface, often moves slightly
faster than the water underneath it. This is because the very wind that drives
the current also drives the plastic. In this sense, the plastic is like a
‘free-rider’ stealing energy from the wind and stealing energy from the
current.

One question you have probably been asking yourself is why
the plastic accumulates in the centre of the gyres. To understand this, you
first have to know about the Coriolis force. There isn’t space here to explain
it in any detail, but basically the Coriolis force is an ‘apparent’ force due
to the rotation of the Earth that acts upon objects and fluids with large-scale
trajectories, by deflecting them to the right in the northern hemisphere and to
the left in the southern hemisphere.

Now, there is another phenomenon called the
Ekman spiral. To understand the Ekman spiral we have to think of the moving
surface of the ocean as a series of layers, each layer driving the one below
it. Firstly, the wind drives the top layer of the ocean, trying to push it in
the same direction as the wind itself. However, due to the Coriolis force and
the friction between one layer and the next, the motion of the top layer of the
ocean is deflected slightly to the right (in the northern hemisphere).
Likewise, each successive layer down is deflected a bit more to the right. As a
result, the net flow of surface water is to the right of the wind direction
(Figure 2). Therefore, in, say, the North Pacific Gyre
which circulates clockwise, the surface water is deflected inwards towards the
centre of the gyre.

Figure 2: Ekman spiral in the
northern hemisphere

Eric van Sebille and colleagues from the University of New
South Wales have been investigating the spread of plastics over the oceans in a
fascinating study. They used data from satellite-tracked drifter buoys, from
the famous long-term Global Drifter
Program, to develop a simulation model for how plastic moves around in the
ocean. Using their model they were able to predict the path of plastics in the
ocean for up to 1000 years.

There are several interesting observations from their study.
For example, releasing plastics from coastlines around the world in their
model, they were able to show clearly how the great Garbage Patches formed
after various times scales ranging from ten to 1000 years. Figure 3 illustrates the simulation after 50 years, which
clearly shows the five major patches plus a sixth one in the Barents Sea. They
were also able to show that the formation of the patches is not local. All the
plastic from all the coastlines ends up in all the patches. Also, over decadal
time scales, the plastic migrates from patch to patch; and over much longer
time-scales – hundreds of years – all the plastic gradually accumulates in the
North Pacific.

Figure 3: simulation of
accumulation of plastics after 50 years, which is about the amount of time we
have been pumping plastic into the sea

The tendency for plastic to accumulate in the gyres explains
why so much of it is found on remote, mid-ocean islands. Places like Kamilo
Beach, Hawaii, or the Midway Islands, are some of the places where more plastic
is found than anywhere on the planet, even though these places are nowhere near
human populations. According to van Sebille’s model, if you are anywhere near
those red patches in Figure 3 you will probably
find large amounts of plastic trash. This not only includes islands near the
Great Pacific Garbage Patch but also Atlantic islands such as the Azores,
Madeira, the Canary Islands, Bermuda and several island chains in the western
Caribbean. The coasts of southwest Iberia and Morocco also don’t get away
lightly. According the model, they are more likely to receive our plastic than
coastlines further north, including those of Britain and Ireland.

In summary, the terms ‘long-term’ and ‘far-reaching’ are not
really adequate for plastic pollution. The fact that plastic won’t degrade and the
fact that the ocean currents transport it all over the globe, suggest ‘eternal’
and ‘infinite’ to be more appropriate. Some estimates say that there are around
5,000,000,000 tonnes of plastic already in existence and, at the present rate
of manufacture, that figure will double every eight years. It is inevitable
that a huge proportion of that plastic will end up in the sea and then get
washed up on coastlines. The only real
solution is to stop manufacturing plastics.
Perhaps if we find a way to do that it soon enough, we might be able to
save the oceans and save the world’s coastlines.

Eric van Sebille and his team have come up with an
interactive website, where you can use the model to ‘spill’ plastic into the
sea at any point around the world’s oceans and see where it goes over the next
ten years: And if you are interested in something a bit more technical,
the original paper can be downloaded here